And, in keeping with their well-known spirit of fair play, the UDenizens continue to "argue" with leo, despite knowing full well that he will not be able to point out their errors again.

Relativistic effects. As RBill explained, they wrote their posts before leo was banned, but back-modification by the loudspeaker makes it appear they are deliberately speaking to a ghost. It's unintentionally funny - imagine actors on a stage, ignoring the cops who have swept in to haul away one of their troupe, and continuing to speak lines to him (and pausing for the responses he is no longer able to make) with nary a pause.